1. Field of the Invention
This invention relates to ceramic coated metal substrates for use in electronic applications. More particularly, this invention relates to such coated substrates having improved processibility characteristics in the manufacture of electronic devices, as for example electronic circuits.
2. Prior Art
In the manufacture of electronic devices, it is standard to mount or form the various electronic components which comprise the circuits of the devices on a substrate. Various materials have been suggested for use as the substrate. For example, relatively large circuits such as those employed in radios, televisions, computers and the like are generally produced on organic substrates, as for example, reinforced thermosetting resins, reinforced phonolic resins and reinforced epoxy resin laminates.
The organic plastic circuit boards have some advantages. For example, such boards are relatively inexpensive, can be manufactured in almost any desired size with flat smooth surface and have reasonably good physical strength. Such organic plastic circuit boards also display a number of deficiencies which greatly limit their utility. For example, these materials are heat sensitive and cannot be exposed to high temperatures, i.e. temperatures in excess of 400.degree. C. Thus, metal circuitry and the like must be formed on the surface of the board using low temperature deposition. Moreover, organic plastic substrates cannot be used with newer methods of applying circuits to non-conductive substrates which are lower in cost and provide improved reliability and electrical accuracy but which demand firing temperatures in excess of 600.degree. C. Another disadvantage of plastic circuit boards is that resistors, capacitors, and the like must be manufactured as discrete components in separate manufacturing operations, and then individually mounted on the board. In addition to the wide variety of manufacturing processes required to make a low temperature circuit board, the use of discrete components severely limits the packing density which can be achieved.
The disadvantages of plastic circuit boards have led to the development of circuit manufacturing techniques which allow direct formation of components (conductors, resistors, capacitors, and the like) on the surface of the substrate, (see M.L. Topfer, "Thick-Film Microelectronics", Van Nostrand Reinhold, NY 1971, and F.N. Sinnadurai, "Handbook of Microelectronics Packaging and Interconnection Technologies", Electrochemical Publications Limited, Great Britain, 1985). The materials used to form these so called process induced components (PIC) are usually prepared in the form of inks comprising metal or ceramic powders, glass powders, polymer binders, and a liquid carrier which are printed on the substrate and heated to drive off the organic binder and fuse the remaining components to the substrate. The firing temperatures required for this process typically range from 600.degree. C. to 950.degree. C., considerably in excess of the degradation temperatures of any organic plastic circuit board. The components prepared at these high temperatures generally exhibit very good reliability and electrical accuracy in addition to being relatively low in cost.
Other circuit board substrates have been fabricated out of ceramic substrates. A variety of ceramic materials are commercially available for use as high temperature substrates. Most notable is aluminum oxide. Examples of such substrates are described in R. Kuzel et al., "Ceramic-Coated Copper Substrates for Hybrid Circuits", Hybrid Circuits, No. 4, pp. 4-9 (1984). These substrates composed of aluminum oxide have the advantage that they have excellent high temperature resistance and may be fired and refired at high temperatures, for example, temperatures of from 600.degree. C. to 900.degree. C. Substrates formed from aluminum oxide are relatively inexpensive when the wafers are small and of simple shape, and have therefore become something of an industry standard. However, these materials also suffer from several disadvantages. An inherent disadvantage of ceramic substrates is that they are quite fragile. This places a significant size limitation on their use and, often times, special fixturing is required to prevent damage while in use. Another disadvantage of ceramic substrates is that they can be quite difficult and costly to machine. These aspects of ceramics generally limit their use to small, rectangular, single sided circuits which have very high reliability and precision.
Still other circuit boards have been fabricated out of porcelain coated metals such as steel and copper clad Invar. Illustrative of such materials are those described in U.S. Pat. No. 4,256,796; L.S. Onyshkevych, et al., "Manufacturing Steps in the Production of Porcelain-Enamel PC Boards", RCA Review, Vol. 42, pp. 145-158 (1981); I. Nobuo, et al., "Thick Film Circuit on Bent Porcelain-Bent Substrate", Int. J. Hybrid Microelectron., 5(2), pp. 1 to 8 (1982);; S. C. Hugh, "Multilayer Thick-Film Circuits on Porcelainized Steel Substrates", Int. J. Hybrid Microelectron., 4(2), pp. 326-330 (1981); K. W. Hang, et al., "Low Expansion Porcelain-Coated Copper-Clad Invar Substrates", RCA Review, Vol. 45, pp. 33-48 (1984); A. N. Prabhu, et al., "Optimization of RCA Porcelain for Compatibility With Thick Films", RCA Review, Vol. 42, pp. 221-237 (1981); E. W. Hughes, "Status Report on Porcelain Enameled Metal Substrates", Ceramic Eng. Soc. Proc. 5, p. 219-220 (1984); R. B. Schabacker, "The Multiplicity of Variations in PEMS", Appliance, 39(6), pp. 76-79 (1982); R. L. Schelhorn, "Metal Core Materials for Thick Film Substrate Applications", Int. J. Hybrid Microelectron., 4(2), pp. 347-352 (1981); K. W. Hang, et al., "High-Temperature Porcelain-Coated Steel Electronic Substrates-Composition and Properties", RCA Review, Vol. 42, pp. 159-177 (1981); and L. S. Onyshkevych, "Porcelain-Enameled Steel Substrates for Electronic Applications", Appliance, 38 (4), pp. 46-49 (1981). Although these boards are not subject to thermal degradation as the organic plastic boards, and are substantially stronger than ceramic, they do suffer from certain process and use deficiencies. For example, when low temperature porcelains are fired to the metal they melt and flow in such a way that a meniscus is formed at all edges. It therefore becomes very difficult to print circuitry over the resulting uneven surface. A further problem with low temperature porcelains is that they soften and reflow at about 600.degree. C. which can lead to distortion of the overlying circuitry. Another problem with porcelains is poor adhesion to the metal substrate during use because of substantial differences between the coefficients of thermal expansion of the porcelain and the metal substrate.
Several attempts have been made to obviate the deficiencies of the porcelain-coated substrates. For example, U.S. Pat. No. 4,256,796 discloses the fabrication of porcelain-coated metal circuit boards wherein the porcelain is a devitrified glass, and U.S. Pat. Nos. 4,358,541 and 4,385,127 describe essentially alkali metal free glass-ceramic coatings for use in the manufacture of circuit boards. While relatively effective, these boards also suffer from several disadvantages. For example, the thermal coefficients of expansion (TCE) of the glass or devitrified glass when matched to the TCE of the metal core results in a substrate with TCE's which are not a good match for TCE's of surface mounted components. For example, in the case of partially devitrified glass, the resultant fired coatings have deformation temperatures greater than 700.degree. C. and thermal coefficients of expansion values greater than 11 ppm/.degree. C. Although the high TCE is considered an advantage for adhesion to the metal core, it is a disadvantage when considering a good match to the surface mounted components (typically alumina or silicon) which have TCEs in the 6 to 8 ppm/C range.
Ceramics containing aluminum oxide, silicon oxide and magnesium oxide are known. For example, cordierite-based glass ceramic materials are described in B. H. Mussler, and M. W. Shafer, "Preparation and Properties of Mullite-Cordierite Composites.", Ceramic Bulletin, Vol. 63, No. 5, pp. 705-714 (1984), and references cited therein. As noted in Ceramic Bulletin, when compared to alumina, presently the most commonly used material, cordierite (2MgO-2Al.sub.2 O.sub.3 -5SiO.sub.2) offers lower dielectric constant and thermal expansion, but has the disadvantage of inferior mechanical properties. Because of the low thermal expansion and inferior mechanical properties of cordierite, it would be expected that the fabrication of electronic devices composed of metal substrates which have relatively high thermal coefficients of expansion coated with cordierite or cordierite based glass ceramic which have relatively low thermal coefficients of expansion would not be feasible because of the large differences in thermal coefficients of expansion.